x/exp/maps: fix tests

The map type argument for the generic function Equal cannot
be inferred from an untyped nil function argument. Provide
a typed nil instead.

Fixes golang/go#51438.

Change-Id: Icf32bb2d66fbb26706e81f1322245b14390741a1
Reviewed-on: https://go-review.googlesource.com/c/exp/+/389374
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
diff --git a/maps/maps_test.go b/maps/maps_test.go
index 8be7f74..6dc6292 100644
--- a/maps/maps_test.go
+++ b/maps/maps_test.go
@@ -52,10 +52,10 @@
 	if !Equal(m1, m1) {
 		t.Errorf("Equal(%v, %v) = false, want true", m1, m1)
 	}
-	if Equal(m1, nil) {
+	if Equal(m1, (map[int]int)(nil)) {
 		t.Errorf("Equal(%v, nil) = true, want false", m1)
 	}
-	if Equal(nil, m1) {
+	if Equal((map[int]int)(nil), m1) {
 		t.Errorf("Equal(nil, %v) = true, want false", m1)
 	}
 	if !Equal[map[int]int, map[int]int](nil, nil) {
@@ -92,10 +92,10 @@
 	if !EqualFunc(m1, m1, equal[int]) {
 		t.Errorf("EqualFunc(%v, %v, equal) = false, want true", m1, m1)
 	}
-	if EqualFunc(m1, nil, equal[int]) {
+	if EqualFunc(m1, (map[int]int)(nil), equal[int]) {
 		t.Errorf("EqualFunc(%v, nil, equal) = true, want false", m1)
 	}
-	if EqualFunc(nil, m1, equal[int]) {
+	if EqualFunc((map[int]int)(nil), m1, equal[int]) {
 		t.Errorf("EqualFunc(nil, %v, equal) = true, want false", m1)
 	}
 	if !EqualFunc[map[int]int, map[int]int](nil, nil, equal[int]) {
@@ -126,7 +126,7 @@
 	if got := len(ml); got != 0 {
 		t.Errorf("len(%v) = %d after Clear, want 0", ml, got)
 	}
-	if !Equal(ml, nil) {
+	if !Equal(ml, (map[int]int)(nil)) {
 		t.Errorf("Equal(%v, nil) = false, want true", ml)
 	}
 }